This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application 2001-019827 filed on Jan. 29, 2001, the entire content of which is incorporated herein by reference.
This invention generally relates to an automatic transmission. More particularly, the present invention pertains to a hydraulic pressure control device for an automatic transmission for automatically changing a shift by controlling a plurality of frictional engagement elements to appropriately control hydraulic pressure for performing the engagement and the release of the frictional engagement elements.
The reduction of the transmitting torque by a release side frictional engagement element in accordance with the increase of the transmitting torque of an engagement side frictional engagement element is required in a shift change. A one-way clutch is a mechanism adapted for reducing the transmitting torque by the release side frictional engagement element. Recently, the one-way clutch has been replaced by a clutch to clutch shift control for achieving the function of the one-way clutch by the hydraulic pressure control to the frictional engagement element.
In the clutch to clutch shift control, when the hydraulic pressure control is not performed appropriately, the output torque of the automatic transmission is suddenly changed and brings about a deterioration of the shift change feeling. For example, when the reduction of the transmitting torque of the release side frictional engagement element is delayed relative to the increase of the transmitting torque of the engagement side frictional engagement element, the output torque is suddenly reduced because of the generation of a so-called inter-locking conditioning condition. On the other hand, when the reduction of the transmitting torque of the release side frictional element relative to the increase of the transmitting torque of the engagement side frictional engagement element occurs too early, the turbine speed is increased (i.e., the turbine rotational number is suddenly increased), thus suddenly reducing the output torque.
Japanese Patent Laid-Open Publication No. Hei 10-153257 discloses a hydraulic pressure control device for an automatic transmission which is adapted to address the difficulties mentioned above. According to this known device, a slip of the release side frictional engagement element is generated by reducing the hydraulic pressure relative to the release side frictional engagement element so that the speed ratio (i.e., the turbine speed/output shaft rotation speed) is within a predetermined range. Simultaneously, the hydraulic pressure relative to the engagement side frictional engagement element is gradually increased. Thus, the speed ratio is sensitively varied in accordance with the engagement force of the engagement side frictional engagement elements, the completion time of preparation for connection (i.e., the time when the sudden increase of the turbine rotational number can be prevented) can be detected based on the speed ratio, and the hydraulic pressure relative to the release side frictional engagement element is suddenly reduced after the detection of the completion time of the preparation for connection.
Generally, the turbine speed becomes unstable when slip is generated and the variation of the turbine speed (i.e., turbine rotation acceleration) appears as the fluctuation of the output torque. The slip amount is susceptible to the variation of the input torque of the torque converter in accordance with the variation of the throttle opening degree of the engine and the variation of the hydraulic pressure characteristics in accordance with the temperature change of the operation oil of the automatic transmission.
The aforementioned document describing the known hydraulic pressure control device for an automatic transmission mentions that the slip amount can be controlled by feedback control, but it is difficult to maintain in a stable manner the slip amount by the usual PI control. As particularly shown by the dashed line in FIG. 12(D), the increase of the hydraulic pressure applied to the engagement side frictional engagement element (i.e., transmitting torque) is delayed due to variations of the clutch stroke and the variation of the hydraulic pressure characteristic in accordance with the temperature change of the operation oil. As a result, as shown in FIGS. 12(A) and 12(B), a time TS during which the speed ratio has to be maintained within a predetermined range xcex94e becomes long. In this case, because the slip amount fluctuates high and low as shown in FIG. 12(B) and the possibility of the fluctuation of the output torque is increased as shown in FIG. 12(C), the shift change feeling is deteriorated.
A need thus exists for a hydraulic pressure control device for an automatic transmission which can smoothly vary the slip amount of the clutch to clutch shift control and maintain a preferable shift change feeling.
According to one aspect, a method of controlling an automatic transmission involves achieving a predetermined shift by maintaining a plurality of frictional engagement elements under an engagement condition or a disengagement condition by controlling hydraulic pressure applied to the frictional engagement elements, with the plurality of frictional engagement elements including a release side frictional engagement element and an engagement side frictional element, generating slip by reducing a transmission torque of the release side frictional engagement element by reducing the hydraulic pressure applied to the release side frictional engagement element which is under the engagement condition before a shift change and is changed to the disengagement condition after the shift change, and performing the shift change by increasing the transmission torque of the engagement side frictional element by increasing the hydraulic pressure applied to the engagement side frictional element which is under the disengagement condition before the shift change and is changed to the engagement condition after the shift change. A reference model is used having a behavior such that an output is determined to be a predetermined slip amount which smoothly varies over an elapse of time with respect to a target slip amount that varies in a stepped manner. The hydraulic pressure applied to the release side frictional engagement element is controlled using a controller which exhibits a behavior corresponding to the reference model by collaboration of a controlled object and the controller for which a hydraulic pressure applied to the release side frictional engagement element is determined to be an input and a slip amount of the automatic transmission is determined to be an output.
According to another aspect, a hydraulic pressure control device is provided in an automatic transmission that includes a plurality of frictional engagement elements adapted to be maintained under an engagement condition or a disengagement condition to achieve a predetermined shift by controlling hydraulic pressure applied to the frictional engagement elements, wherein the plurality of frictional engagement elements including a release side frictional engagement element and an engagement side frictional element, with slip being generated by reducing a transmission torque of the release side frictional engagement element through reduction of the hydraulic pressure applied to the release side frictional engagement element which is under the engagement condition before a shift change and is changed to the disengagement condition after the shift change, and with the shift change being performed by increasing the transmission torque of the engagement side frictional element through increase of the hydraulic pressure applied to the engagement side frictional element which is under the disengagement condition before the shift change and is changed to the engagement condition after the shift change. The hydraulic pressure control device includes a reference model exhibiting a behavior such that an output is determined to be a predetermined slip amount which smoothly varies over time relative to a target slip amount that varies in a stepped manner, and a controller which controls the hydraulic pressure applied to the release side frictional engagement element, the controller having a behavior corresponding to the reference model by collaboration of a controlled object and the controller for which a hydraulic pressure applied to the release side frictional engagement element is determined to be an input and a slip amount of the automatic transmission is determined to be an output.